1. Field of the Invention
The present invention relates to a single mode optical fiber which has high reliability of a hydrogen-proof characteristic and is suitable for wavelength division multiplexing (WDM) transmission in 1310 nm to 1625 nm, a method of evaluating whether an optical fiber is of that sort, and a method of fabricating such an optical fiber.
2. Related Background Art
Much development has been conducted on optical fibers used for WDM transmission. At first, development of optical fibers was focused on a 1.55-μm band (e.g., in the wavelength range of 1535 to 1570 nm), in which the transmission loss of an optical fiber composed of a silica glass becomes a minimum; at present, studies to broaden the wavelength range for use to 1310 nm through 1625 nm are being conducted.
On the other hand, since a conventional optical fiber is often contaminated with OH groups as an impurity, which has an absorption peak at the wavelength of approximately 1385 nm, transmission in the vicinity of this wavelength is difficult. Therefore, for broadening the wavelength range of WDM transmission to 1310 nm through 1625 nm, those absorption losses must be made to be as small as possible. Japanese Patent No. 3301602 discloses an optical fiber having a low loss at 1385 nm, and its fabrication method.
Now, it is known that there is a phenomenon that the transmission loss also increases when hydrogen diffuses into an optical fiber. One cause of the increase in the transmission loss due to hydrogen is considered to be a following mechanism (Namihira, NIKKEI ELECTRONICS, 1984. 12. 3, PP. 233–248, K. H. Chang et al, Feb. 21–26, 1999, OFC/IOOC'99).
The core of an optical fiber is generally doped with germanium, and has a higher refractive index than that of the surrounding cladding. In a drawing process of an optical fiber being drawn from an optical fiber preform, the optical fiber preform is exposed to a high temperature and a high tension, and is rapidly cooled with glass structures broken. Hence, structural defects as indicated by the formula (1) are considered to be generated in the core.≡Si—O—Ge—O—Si≡→≡Si—O.+.Ge—O—Si≡  (1)
That is, bonds of Ge—O with a weak bonding strength are broken to form non-bridging oxygen hole centers (NBOHC; ≡Si—O.) that are one type of paramagnetic defects, and Ge—E′ (.Ge≡). Diffusion of hydrogen into the optical fiber brings about a reaction of the NBOHC and hydrogen, causing the increase in the transmission loss. Specifically, a reaction indicated by the formula (2) forms OH groups, which involves an increase in the absorption loss. This reaction is known to occur at room temperature.2≡Si—O.+H2→2≡Si—OH  (2)
The density of the residual NBOHC in the optical fiber is known to depend largely on the cooling rate at drawing. A higher cooling rate is prone to the more residues. Hanabusa, Ceramics, 21 (1984), No. 9, pp. 244–252 discloses that the density of the paramagnetic defects can be observed by the electron-spin resonance (ESR) method. US Patent Publication No. 20040475764 shows that the electron spin density of NBOHC is measured by ESR method, and the hydrogen resistance of an optical fiber is evaluated based on the obtained values.
Methods of decreasing the NBOHC density in an optical fiber to suppress the transmission loss deterioration includes one in which an optical fiber during or after drawing is exposed to an atmosphere containing hydrogen or deuterium.
However, the hydrogen treatment of exposing an optical fiber to an atmosphere containing hydrogen is not preferable because the increase in the transmission loss occurs due to formation of OH groups although NBOHC vanishes as shown by the formula (2). On the other hand, the deuterium treatment using deuterium instead of hydrogen brings about a reaction indicated by the formula (3) in an optical fiber.2≡Si—O.+D2→2≡Si—OD  (3)
That is, since the deuterium treatment forms OD groups, the absorption due to OH groups does not occur. Since the OD groups do not have a large absorption peak in 1310 nm to 1625 nm, the transmission loss in the wavelength range is little affected. Therefore, the deuterium treatment of an optical fiber is an effective means to improve the hydrogen-proof characteristic of the optical fiber.
213 Incidentally, although a deuterium-treated fiber is supposed to cause no transmission loss at approximately 1385 nm due to OH groups even if it is exposed to hydrogen after the treatment, the transmission loss at the wavelength of approximately 1400 nm increases at times as shown in FIG. 3. FIG. 4 shows changes with lapse time of the transmission losses at the wavelength of 1400 nm after a deuterium treatment recited in the embodiments of the present invention. The absorption peak is unstable and prone to decrease with lapse time; however, the observation in a very long lapse time is needed for guaranteeing the fiber quality, thus presenting a great obstacle to fabricating an optical fiber. Therefore, when the deuterium treatment is conducted, it is desirable that the increase in the transmission loss at the wavelength of approximately 1400 nm do not occur.
An object of the present invention is to provide an optical fiber with a small increase in the loss at the wavelength of approximately 1400 nm involved in a deuterium treatment. Further, an object of the present invention is to provide a method of evaluating whether or not an optical fiber is that generating an increase in such loss, and a method of fabricating an optical fiber with a small increase in such loss.